Liquid crystal displays (LCD) are widely applied in electrical products, such as digital watches, calculators, and the like. Moreover, with the advance of techniques for manufacture and design, thin film transistor liquid crystal display (TFT-LCD) has been introduced into portable computers, personal digital assistants, and color televisions, and has gradually replaced the conventional cathode ray tube displays.
Transmission LCDs have been the main field of development. Generally, a light source, called a back light, of a transmission LCD is located behind the display. Hence, the material used for the pixel electrodes has to be a transparent conductive material such as indium tin oxide (ITO). The back light of a transmission LCD is the most power-consuming part. However, the widest application of LCDs is portable computers and communication products, for which batteries are the main power supply during use. Therefore, how to decrease the power consumption of an LCD is a main direction in LCD product development. Moreover, the reflection of the transmission LCD when used in a bright environment reduces the contrast to form a fuzzy image.
A reflective LCD is a solution to the problems mentioned above. The light source of a reflective LCD is located outside the LCD; therefore, a reflective layer is needed to reflect the light. Conventionally, pixel electrodes are used as the reflective layer. The material used for the pixel electrodes has to be a reflective conductive material such as metal aluminum. To achieve a better reflection, the surface of the pixel electrodes is uneven. However, there is still an unsolved problem for the reflective LCD. That is, when the intensity of light from the outside light source is not strong enough, the reflective LCD cannot display a clear image. Therefore, the transflective LCD has become the next target of research and development. The pixel electrodes of some transflective LCDs are aluminum plates having at least one opening filled with ITO. Therefore, when outside light intensity is not strong enough, the back light can be turned on to serve as a light source.
Typically, a scattered rough surface is formed to serve as the surface of the reflective layer. The height difference of the rough surface is about 0.5 to 1.5 μm. Such a height difference affects the arrangement of the liquid crystal molecule to reduce the image quality. FIG. 1 is a cross-sectional drawing of the conventional liquid crystal display. Referring to FIG. 1, the liquid crystal display comprises a top substrate 20 and a bottom substrate 10. A liquid crystal layer is disposed between the top substrate 20 and the bottom substrate 10. A reflective layer 12 made of resin is formed over the bottom substrate 10. The height difference existing in the surface of the rough layer 12 changes the cell gap. The cell gap in the protruding region 14 in the rough layer 12 is less than the concave region 16 in the rough layer 12. The reflective efficiency is related to the retardation (R) of the liquid crystal cell. The retardation (R) of the liquid crystal cell is related to the change value (Δd) of the cell gap and the birefringence (Δn) of the liquid crystal. Typically, the birefringence Δn of the liquid crystal is about 0.06 to 0.1. Therefore, the change value Δnd is 0.06 μm to 0.15 μm (Δndj to Δndi) if the change value (Δd) of the cell gap is 0.5 μm to 1.5 μm.
The perfect change value Δnd of the retardation is less than 0.06 μm for the reflective twisted nematic mode (RTN mode) and mixed twisted nematic mode (mixed mode). Such change value Δnd reaches a reflective efficiency from 95% to 100% no matter what the value of the twisted angle is. However, the height difference existing in the conventional reflective layer enlarges the change value Δnd to reduce the reflective efficiency, from an ideal 100% to 60%. The low reflective efficiency cannot efficiently reflect ambient light to the user to make a clear image.